1. Field of the Invention
The present invention relates to thermal conductivity control devices, and more particularly, to thermal conductivity control devices that reduce the heat transfer rate from a heat source to a heat sink below a predetermined temperature.
2. Description of Related Art
Vehicles rely on batteries for power when power is not available from onboard electrical generators or an external source. Typically, vehicles have used conventional batteries with lead-acid cells or nickel-cadmium cells to meet onboard power needs when onboard generators and external sources are not available. Developments in vehicle systems have led to greater power requirements, and vehicle designers have therefore turned to high energy chemistry batteries for vehicular power storage. Lithium metal and lithium ion chemistry batteries, such as lithium cobalt oxide, have gained widespread acceptance in vehicular applications owing to their relatively small size and weight for a given amount of power storage. High energy chemistry batteries are desirable in automotive and aerospace applications because of their high energy density relative to conventional battery chemistries, such as nickel cadmium or lead acid chemistries.
High energy batteries generate power through an exothermic reaction. The heat generated by the exothermic reaction needs to be dissipated from the battery. Vehicles therefore typically include a thermally efficient conduit coupling the battery to the environment external to the vehicle to dissipate heat while the battery is in operation. High energy batteries can also have a minimum operating temperature below which battery performance degrades. They therefore rely on internally generated heat or a battery powered external heating element to maintain the temperature of the battery above a minimum operating temperature. However, under certain conditions, the highly efficient conduit used to cool the battery during operation also dissipates heat generated to keep the battery above its minimum operating temperature, thereby reducing available battery power and/or reducing battery life. Vehicles operating in cold ambient temperature environments are particularly susceptible to this problem.
Conventional methods and system for battery cooling have generally been considered satisfactory for their intended purpose. However, there is a need in the art for a thermal conductivity control device capable of reducing conduction path effectiveness during operation in cold ambient temperature environments. There is a further need for controlling thermal conductivity effectiveness in cold ambient temperature environments that is easy to make and use, and readily adaptable for use in existing battery installations. The present invention provides a solution for at least one of these needs.